Various resins have heretofore been used as materials for containers of seasonings, oils, beverages, cosmetics and detergents according to the kind of contents with which the containers are filled and to the purpose which the containers are used.
Because of its excellent mechanical strength, heat resistance, transparency and gas barrier properties, polyethylene terephthalate is a useful material for forming containers to be filled particularly beverages such as fruit juices, cooling drinks and carbonated drinks.
Such polyethylene terephthalate as referred to above may, for example be prepared by esterifying terephthalic acid or its ester-forming derivative with ethylene glycol or its ester forming derivative in the presence of an esterification catalyst, and polymerizing the esterified product in liquid phase in the presence of a polymerization catalyst, followed by solid phase polycondensation. In general, polyethylene terephthalate thus prepared is fed to a molding machine such as an injection molding machine and molded into a blow-molding preform, and thereafter the preform is inserted into a mold of a predetermined shape and stretch blow-molded into a blown container, and if necessary followed by heat setting.
However, known polyethylene terephthalate obtained by the conventional procedure as mentioned above contains oligomers such as cyclic trimer, with the result that the oligomers attach to the inner surface of blow-molding mold and contaminate the mold therewith or attach to the vent portion of mold of the injection machine as mentioned above and contaminate the mold therewith.
Such contamination of the mold as mentioned above forms the determining cause for surface roughness or chalking of the resulting bottle. If the bottle obtained is chalked, either partly or wholly, it has to be discarded. Owing to such circumstances, in molding bottles out of the known polyethylene terephthalate, there was such a serious problem that contaminants attached to the mold must be removed therefrom at frequent intervals and hence productivity of the bottle markedly decreases.
In light of such existing circumstances as mentioned above, the present inventors extensively studied with the view of obtaining polyethylene terephthalate, by the use of which the contamination of the mold therewith is difficult to occur, and have found that the contamination of the mold at the time of molding polyethylene terephthalate is mainly attributable to an increased amount of oligomers such as cyclic trimers contained in the polyethylene terephthalate, said oligomers being formed in large amounts at the time when said polyethylene terephthalate is molded.
The present inventors further studied on the basis of the above-mentioned finding, and eventually have found that the increase in amount of the oligomers in polyethylene terephthalate at the time of molding may be inhibited by bringing the polyethylene terephthalate into contact with water, and they have accomplished the present invention.
Furthermore, the present inventors have found that there is an intimate relationship between a rate of polycondensation at the time when polyethylene terephthalate used for molding is subjected under specific conditions to solid phase polycondensation treatment and an increasing amount of oligomers formed at the time of molding, and that if the rate of polycondensation treatment of the polyethylene terephthalate is less than a specific value, the contamination of the mold at the time of molding is difficult to occur, on the basis of which finding the present invention has been accomplished.
In this connection, Japanese Patent L-O-P Publn. No. 25815/1984 discloses a process which comprises treating particulate polyethylene terephthalate, prior to solid phase polycondensation thereof, with water vapor heated to above 110.degree. C. so as to crystallize the polyethylene terephthalate.
Further, Japanese Patent L-O-P Publn. No. 219328/1984 discloses a process for preparing polyesters of high polymerization degree, which comprises a step of moisture conditioning a polyester having ethylene terephthalate units as the main repeating units and having an intrinsic viscosity of at least 0.4 dl/g and a density of less than 1.35 g/cm.sup.3 so as to have the moisture content of at least 0.2% by weight, a step of precrystallizing the polyester thus treated, and a step of solid phase polymerizing the precrystallized polyester at a temperature of above 180.degree. C. and below 240.degree. C. in an inert gas atmosphere or under reduced pressure.